The process of using grit, especially steel grit, as a blast media for cleaning steel goes back to the early 1900's, but it was not until the mid 1980's that steel grit began to be used in mobile applications. Some of the first places steel grit was used was for blast cleaning of steel tanks of all types and sizes, including nuclear torus vessels, water, and fuel tanks. When used inside a tank, moisture was typically not a problem, as the tank itself provided protection from rain.
When steel grit began to be used on the exteriors of tanks and then bridges, the exposure to the environment made contamination from sudden rainstorms and other water sources a problem. Steel grit by its nature requires absolutely dry conditions, including the use of compressed air dried to a −10 degree dew point for abrasive blasting operations where condensate would cause corrosion of the steel grit particles.
When moisture comes in contact with steel grit particles, the moisture causes the particles, which are generally size of a grain of sand, to stick together until exposure to air begins the process of corrosion, leading to rusting. When rusting occurs, the small steel grit particles are bonded together by the corrosion process as the grit dries and rust forms. The corrosion process causes clumping in the steel grit as the steel grit particles literally rust themselves together, forming irregularly shaped “rocks” of thousands of corroded particles.
The Economics of Using Steel Grit
In the process of abrasive blasting of bridges, tanks, ships and other steel objects, steel grit is becoming a popular media for a number of reasons. The primary reason is economics. The steel grit particles can be recycled up to 50 or more times with non-metallic particles being removed on each pass through the recycling machine. In addition to the recycling benefit, the density of steel grit is roughly 2.5 times greater than sand or coal slag, so the impact of the steel grit on a steel structure is greater, meaning that more work is accomplished each time a particle hits the surface.
The steel grit abrasive blasting process is especially popular where hazardous paint coatings must be removed, which creates a quantity of waste that must then be disposed of as hazardous waste by law. By using steel grit, which gets recycled each time with all non-metallic hazardous material being removed through the recycling machine, the volume of waste can be reduced to roughly 1% of what would be created if non-recyclable medias like sand or coal slag are used. The recycling dramatically reduces the volume of waste that needs to be disposed of thereby significantly reducing the cost of proper hazardous waste disposal. These economic benefits are what justifies the cost of steel grit recycling machines.
Because the cost of steel grit per ton is many times that of sand or slag, the steel grit must be recycled again and again to gain the economic benefits for the user, while at the same time reducing the volume of waste taken to disposal sites. Thus, when the grit falls to the containment surface or ground, the grit must be quickly recovered, usually using a vacuum device that pulls the grit back to the recycling machine.
Vacuuming or Gravity Recovery of Steel Grit
It is common today to use powerful vacuums driven by large diesel engines to recover the steel grit, whether the steel grit is collected on the ground, on a containment surface, or into some sort of collection hopper. In the recovery process, the steel grit can become mixed with flowing water from rain, which turns the mixture into damp or wet steel grit, thereby making it even heavier than the normal density of 265 lbs. per cubic foot. The added moisture additionally causes the steel grit to become sticky, where the granular steel grit no longer flows as it would at an angle of repose of between approximately 30 and 40 degrees.
Because the steel grit is so valuable, costing up to $900/ton, the operator recovers the grit back to the recycling machine even though he knows the free moisture will cause clogging and eventual clumping as the grit turns to rust. While vacuuming the steel grit for recycling, large water droplets are typically removed from the steel grit. However, enough moisture content on the grit itself remains to cause the rusting and clumping processes. In the process of vacuuming, any opening or wear of the vacuum hose can also allow water to enter the system, causing further moisture problems.
If the wet grit is allowed to sit for a prolonged period of time (e.g., a few days), the grit can become so hard that it must be removed using a jack hammer or other impact device. When sitting in a hopper, the water naturally drains to the bottom and can be drained off if a stainless steel filter screen at the bottom allows for drainage. However, moisture content residing on the surface of the steel grit that is not removed during the vacuuming process typically remains long enough to cause the rusting and clumping processes to occur.